1. Field of the Invention
The present invention relates generally to an acoustooptic recording apparatus, and, more particularly, to a high speed, wide bandwidth optical system for reading out a serial electrical signal as parallel optical data.
2. Description of the Prior Art
High speed recorders for recording a serial line of electrical data as a spatial array of optical data are important in many fields. For example, in imaging applications, they are used to reproduce a picture or image that has been transmitted electrically over large distances. In addition, they are used in a wide variety of signal processing systems including systems for performing spectrum analysis.
One conventional prior art technique for high speed data recording is with the use of a laser scanner. In such systems, a laser beam is caused to scan across a recording medium by a suitable mechanical scanning apparatus such as a system of rotating mirrors or prisms. By turning the laser beam on and off or otherwise modulating it with the signal to be recorded as it is scanned across the recording medium, the data can be optically reproduced on the medium.
Such systems, being mechanical in nature, suffer from obvious limitations. For one thing, they are somewhat limited in scanning speed as the mirrors or prisms tend to distort at excessive speeds of rotation. Also, they have bearings and other mechanical components which eventually wear out. In addition, the scanning apparatus frequently must be maintained in a vacuum to eliminate air turbulence problems caused by the mirror rotation, and this makes the system unweildy and not very portable. Finally, it is essentially impossible to make each facet of the scanner exactly the same, and, as a result, as an array of lines are put down in a raster format, banding and line jitter will occur, at least to some extent.
More recently, the prior art has developed a solid state scanner which requires no moving parts and, therefore, does not suffer from many of the above-described inadequacies. Typically, these systems employ an acousto-optic cell to replace the rotating mirrors or prisms. Specifically, a laser beam is first appropriately modulated by the electrical signal to be spatially recorded and is then passed through the cell and focused to a point on the recording medium. A sinusoidal acoustic signal is also passed through the cell, and, as is understood in the art, this will cause the laser beam to be diffracted at a given angle. By then changing the acoustic signal frequency in a continuous manner, the laser beam will be scanned across the recording medium to form a line of points representative of the input electrical signal by which the beam is modulated. By indexing the recording medium, an entire raster or series of lines of data can be recorded.
This system, while having the advantage of being fully solid state, still suffers from several shortcomings. For one thing, it is, for practical purposes, limited in the number of spots that can be placed along a single line. Specifically, in such systems, there is a trade-off between the number of points that can be placed along a line and the scan rate (i.e., the so-called .beta..tau. product). For example, for a commercially available 100 megahertz bandwidth device, although 1,000 resolvable spots can be recorded at low data rates, only 500 spots can be recorded at 50 million spots per second, and only 2 resolvable spots can be recorded at 100 million spots per second. Also, in these systems, there is a loss in duty cycle due to the time required for the first acoustic signal to pass through the cell before the next signal can be sent. Further, the spot position accuracy obtainable with such systems are not as great as might be desired. Finally, the optical quality of the acousto-optic cells used in this type of system must be quite good or distortions will result.